Compared with a 3G technology, a Long Term Evolution (LTE) system receives much concern and is highly praised due to the characteristics of high data rate, flexible spectrum configuration mode and packet transport mode, low delay, wide coverage domain, high downward compatibility and the like. The LTE system simultaneously defines two different duplexing modes, namely Frequency Division Duplexing (FDD) and Time Division Duplexing (FDD). Here FDD performs transmitting and receiving over two separated symmetric frequency channels, and separates transmitting and receiving channels by a protective frequency band, so FDD must distinguish uplinks and downlinks by means of paired frequencies, resources in a single direction thereof are successive temporally. When supporting symmetric services, FDD can fully utilize uplink and downlink spectra. However, when supporting asymmetric services, the spectrum efficiency will be greatly reduced. In a conventional cell wireless communication system, downlink channel of a typical FDD mode system is usually allocated to a higher frequency band. This is because the higher the frequency band corresponding to a signal is, the greater the energy loss caused by path loss is. In a downlink direction, a Base Station (BS) may compensate the path loss by using higher transmitting power, while for an uplink direction, a mobile station end will reduce the transmitting power to the greatest extent due to limitations of device size and design cost, therefore, a downlink channel is usually allocated to the higher frequency band in an FDD system.
Because an original dedicated radio system occupies conventional frequency resources, different operator networks are improperly configured and there exists the problems of transmitter setting, geographical location overlapping, Electro Magnetic Compatibility (EMC) and intentional interference, a wireless communication system has various interferences such as same frequency interference, adjacent channel interference, out-of-band interference, inter-modulation interference and blocking interference, etc. The leading interference therein is the interference caused by resource reuse. In an actual wireless communication system, usually a plurality of users shares some communication resources, so when a plurality of channels transmit data by using an identical resource such as a frequency subcarrier and a time slot, the channels will interfere with each other. Particularly, as for downlink transmission of a multi-cell and multi-user Multiple-Input Multiple-Output (MIMO) system, when a certain BS (transmitter) sends information to a user (receiver) in a cell where the BS is located, because a frequency reuse factor of the system is 1, the BS will interfere with users in other cells, this interference being Inter-Cell Interference (ICI). Because the BS serves a plurality of users by using the same frequency resource at the same time, Inter-User Interference (IUI) may exist between the users in the cells. Due to the coexistence of ICI and IUI, the system performance will be seriously limited by the interference. Therefore, an effective interference suppression technology is an objective of a prolonged endeavor of each research institute. An interference alignment technology is an emerging novel interference suppression mode with great potential in recent years, which may greatly improve the system capacity. The interference alignment technology evaluates the system capacity by using the Degree of Freedom (DoF) (also called as multiplexing gain), the accuracy of evaluation is improved along with increase of a Signal to Noise Ratio (SNR). Obviously, the emergence of the interference alignment technology creates a new direction for interference suppression, and the interference alignment technology also attracts wide attention from various academic institute and research institute at home and abroad.
The basic idea of interference alignment is to co-design a precoding matrix and a receiving matrix between each transmitter and receiver, to limit an interference signal within a subspace of a receiving signal space, and to reserve another interference-free signal subspace for data transmission. The implementation premise of the interference alignment technology is that the transmitter and the receiver need to obtain global Channel State Information (CSI). In an actual communication system, a TDD system may obtain ideal Channel State Information at Transmitter (CSIT) by using channel reciprocity. However, an FDD system does not have the channel reciprocity, and CSIT cannot be obtained directly by channel estimation. Usually, as for the FDD system, the receiver performs channel estimation, and then feeds back the obtained CSI to the transmitter via a feedback channel. This feedback mode may form non-ideal CSIT due to the following factors:
channel estimation error: a CSI estimation algorithm cannot completely obtain ideal information of a current channel, a certain error exists inevitably, and the CSIT is non-ideal after feeding back to the transmitter;
channel delay: channel measurement is performed at the receiver, it will take some time to feed back a measurement result to the transmitter, and in a time-varying channel, when feedback information is transmitted to the transmitter, a channel state has been changed, thus causing that the Channel State Information obtained at the transmitter is delayed; and
limited feedback channel capacity: the limitation of a feedback channel capacity affects the accuracy of the CSIT, and at this moment, the transmitter may only obtain part of the CSIT.
Therefore, in an actual communication scenario, due to the existence of the problems of time-varying characteristics of a channel, capacity limitation of a feedback channel, feedback delay and the like, the CSIT is non-ideal. The performance obtained based on interference alignment of the non-ideal CSIT will also decrease inevitably. Therefore, it is very important to study the influence on the interference alignment caused by the non-ideal CSIT to propose a new interference alignment improvement solution, and reduce the influence on the system performance caused by the non-determinacy of the CSI.
From the perspective of information theory, a Dirty Paper Coding (DPC) theory proves that in an interfered system, if the transmitter can accurately learn of an interference signal, the channel capacity of an interfered system may be identical to that of an interference-free system by means of certain precoding processing at the transmitter. Because the DPC theory is difficult to be applied to an actual system, some suboptimal precoding technologies emerge. One class is a precoding technology based on real-time channel processing such as channel inversion, channel Block Diagonalization (BD), etc., where it is necessary for the transmitter to learn of all or part of CSI, and at this moment, the feedback quantity of channel information is larger, which is not in favor of practical application. The other class is a codebook-based precoding technology. The codebook-based precoding technology pre-learns of fixed codebooks at both the transmitter and the receiver, the system selects an appropriate precoding vector from the fixed codebooks to precode a transmitting user according to Channel Quality Information (CQI) which is fed back, and the feedback quantity may be reduced by codebook selection due to limited channel capacity. Therefore, a codebook selection-based precoding solution has a higher practical value due to small feedback quantity.
An effective solution has not been proposed yet at present for the problem in the existing technology that ideal Channel State Information can not be obtained at the transmitter in an LTE-FDD system.